A human body is composed of millions and trillions of cells that combine to make tissues helping in the functioning of vital organs. A cell’s internal structure is maintained throughout its life because its plasma membrane is partially permeable to molecules entering it. There are many molecules that cannot easily pass through the lipid bilayer, thus requiring external aid to get in.
Therefore, nature possesses various mechanisms to proceed with cellular functions, such as active and passive transport, different diffusions, osmosis, respiration, etc. This article will help you investigate the differences between facilitated diffusion and active transport. So, shall we start?
Comparison Table
| Factors | Facilitated Diffusion | Active Transport |
| Direction of Transport | From high to low medium | From low to high medium |
| Energy Requirement | No | Yes |
| Nature | Passive | Active |
| Transport Protein Types | Carrier proteins | Carrier proteins and pumps |
| Molecules Type | Large and polar | Small and ionized |
| Specificity | Yes | Yes |
| Saturation | Yes | Yes |
What is Facilitated Diffusion?
It is a type of passive transport that enables the molecule movement across a cell membrane, from an area of higher concentration to a place of lower concentration – along the concentration gradient. Facilitated diffusion is used to move large or polar molecules to pass through the cell membrane independently. The process involves the use of transport proteins, such as channel and carrier proteins. These transport proteins are very specific, which means they only allow certain molecules to pass through the lipid barrier.
What Factors Affect Facilitated Diffusion?
Brownian motion is actively involved wherever fluid diffusion occurs; hence, various factors affect the processing of facilitated diffusion. These are:
Concentration of Molecules: The process will kick start from an area where there is a higher concentration of molecules to a place of lower concentration.
Temperature: The movement of molecules along the concentration gradient increases with the increase in temperature.
Molecule Size: Smaller molecules easily diffuse through the membrane than larger molecules. It is because these are lighter in weight and can float immediately.
Distance of Diffusion: Facilitated diffusion is faster at an area of less space than a larger one. For example, gas molecules diffuse faster through a thin surface than thick.
Examples of Facilitated Diffusion
Facilitated diffusion is unlike simple diffusion, which only allows smaller or non-polar molecules to pass through. This process is necessary for polar and charged particles to shift across the membrane, thus helping the cell’s proper functioning. Below are some examples of facilitated diffusion.
Transport of Glucose
Glucose is one of the essential polar molecules that cannot cross the lipid bilayer on its own and require a pathway. Cells have special carrier proteins called “glucose transporters” that facilitate the movement of these molecules. The most common glucose transporter is GLUT1, found in most cell types and is responsible for basal glucose uptake. Other glucose transporters are also present in different cell types, such as GLUT2 in the liver allows glucose to be transported in and out of this organs as needed.
Transport of Ions
Facilitated diffusion also helps transfer charged molecules besides the polar ones. Ions, such as sodium, calcium, and potassium naturally involved in various biological processes, including nerve conduction, muscle contraction, and pH regulation. However, these ions are charged and cannot easily cross the cell membrane. Therefore, cells use specific ion channels and transporters to facilitate movement. For example, the sodium-glucose cotransporter is responsible for transporting sodium and glucose across the membrane in the small intestine and kidney tubules.
Transport of Proteins – Amino Acids
We know that amino acids are the building blocks of different types of proteins, essential for the growth and repairmen of tissues. However, most of these large molecules are polar and unable to cross the cell membrane. Just like cells have transporters for glucose, they also have amino acid transporters, including the SLC1A5 transporter. It is responsible for transporting large neutral amino acids, including leucine and phenylalanine.
Transport of Water
Although water molecules are small, they cannot cross the hydrophobic lipid bilayer due to their polar nature. Therefore, cells have special channels for water transport called Aquaporins. Different types of aquaporins are involved in water transport across the membrane; for example, the aquaporin-2 channel is found in the kidney and plays a critical role in water reabsorption.
What is Active Transport?
The transport of molecules by carrier proteins can be active or passive. Active transport requires energy in the form of ATP to move solutes against their concentration gradient, i.e., from a region of low concentration to an area of high concentration. As mentioned, active transport involves transport proteins; however, it differs from facilitated diffusion because of requiring energy. Transporters or pumps are the specific membrane proteins helping the ions or molecules move against the concentration gradient.
What Factors Affect the Rate of Active Transport?
Several factors greatly influence the rate and functioning of active transport. These are:
Nature and No. Of Transporters: As transporters help molecules move from one location to another, more of these transporters will result in a faster rate as more molecules can be moved simultaneously.
Temperature: It directly affects the generation of ATP; thus, increased temperature will lead to rapid active transport. However, a high temperature can denature the transport proteins and decrease the rate.
ATP Availability: As the process requires energy in the form of ATP, more ATP molecules will increase the molecules’ transfer process.
pH: Proteins are sensitive structures that can be denatured under the influence of changed pH. Thus, pH plays an important role in the transport of molecules through active transport. For a maximum rate of functioning, optimal pH conditions are required.
Molecule Size: Smaller molecules will tend to diffuse faster through the transporters against the concentration gradient and vice versa.
Types of Active Transport
Several types of active transport take place in a body for physiological purposes, including primary and secondary active transport, vesicular or bulk transport, bacterial lactose transport, etc. But, we will discuss the major ones that are continuously occurring in the body.
Primary Active Transport
It is the most common type of active transport involved in directly using ATP to cross molecules across the cell membrane of plant and animal cells. A typical example is the sodium-potassium pump, which creates a charge gradient that can be used to move other molecules. Two potassium ions are pumped in for every three sodium ions that are transported out of the cell.
Secondary Active Transport
Secondary active transport involves using an electrochemical gradient to move molecules across the membrane. This process is driven by the movement of ions across the membrane, developing a charge gradient to move other molecules easily. The most common type of secondary active transport is the sodium-glucose transporter. It utilizes energy stored in the sodium gradient to transport glucose into the cell against its concentration gradient.
Vesicular or Bulk Transport
It is another type of active transport, and as the name indicates, it involves the formation of vesicles. There are two main types of vesicles formed, further categorizing the transportation of molecules. These are:
Endocytosis: It helps to engulf larger particles in the cell by forming a vesicle.
Exocytosis: This process involves the fusion of vesicles to the cell membrane, so the vesicular content or molecules are secreted out of the cell.
Examples of Active Transport
Animal and plant physiology is full of examples of active transport. A few of them are:
In Animals
- Movement of Na and K through sodium-potassium pump
- Movement of Calcium ions for cardiac muscles
- Phagocytosis of bacteria by macrophages
- Releasing of antibodies through vesicles
- Movement of amino acids along the intestinal tract
In Plants
- Calcium ions utilize ATP to move across cells
- Chloride ion transportation from the cytosol to the vacuoles for nutrition and growth factors
- Transportation of minerals from the stem to other parts of the plant
Similarities between Facilitated Diffusion and Active Transport
While the two processes are different from each other, they have a few points in common, discussed below:
- The main purpose of facilitated diffusion or active transport is to transfer the molecules in and out of the cell membrane.
- Both processes transport sugar molecules, salt, and ions across the membrane.
- Carrier proteins or transporters help both processes move different molecules across the cell membrane.
Facilitated Diffusion vs. Active Transport – Key Differences
Now, let us look at the main differences between the two processes that are highlighted below:
Definition
Facilitated Diffusion
It is a process of transfer of larger molecules from an area of higher concentration to an area of lower concentration with the help of transmembrane proteins (transporters).
Active Transport
On the contrary, active transport is the shifting of molecules from an area of low concentration to an area of higher concentration with the help of energy.
Energy Requirement
Facilitated Diffusion
As facilitated diffusion is a passive process, it does not require energy and can easily move along the concentration gradient.
Active Transport
However, active transport is entirely an energy-dependent process that will always require it in the form of Adenosine Triphosphate (ATP) to continue processing.
Movement of Substances
Facilitated Diffusion
As a passive process, facilitated diffusion moves substances down their concentration gradient.
Active Transport
In contrast, active transport is a dynamic process that moves the substances against their concentration gradient.
Rate of Transport
Facilitated Diffusion
This process reaches a maximum rate when all the transporters are occupied, and the process limits there.
Active Transport
However, the molecules maintain a constant rate of active transport even when the concentration gradient is low.
Examples of Transporters
Facilitated Diffusion
GLUT transporters, ion transporters, aquaporins, and amino acid transporters like LAT1, ASCT1, and ASCT2 are common examples of transporters in facilitated diffusion.
Active Transport
Similarly, the common transporters of active transport are sodium-potassium ATPase transporters, mitochondrial ATP synthase, sodium-glucose transporter, etc.
Bottom Line
Facilitated diffusion and active transport are two important mechanisms of cellular transport that differ in how they move substances across the cell membrane. Facilitated diffusion is a passive process that does not require energy and moves substances down their concentration gradient. In contrast, active transport is an energy-dependent process that moves substances against their concentration gradient. These processes are highly selective and play important roles in maintaining cellular homeostasis.
Anna has completed her degree in Pharmacy from the University of Hawaii. She is serving as a research assistant in a pharmaceutical company. She had a great interest in writing blogs, traveling to different parts of the US, and trying delicious recipes in her spare time.